Is it possible to create a passwall through the Stone Golem body with the Passwall spell?
This should not work
The passwall spell states:
A passage appears at a point of your choice that you can see on a wooden, plaster, or stone surface (such as a wall, a ceiling, or a floor) within range, and lasts for the duration. You choose the opening's dimensions: up to 5 feet wide, 8 feet tall, and 20 feet deep. The passage creates no instability in a structure surrounding it.
Fifth edition uses natural language (standard English) when describing things so it is left to us to rule what a "surface" is. Lexico defines "surface" as:
The outside part or uppermost layer of something.
So very technically it would seem that a Stone Golem would work, however the spell lists examples of surfaces "walls, ceilings, floors" these are very similar surfaces and they are all objects, not creatures. If the spell wanted creatures to be included as an option it would have said so.
Another thing that I believe is true is that surfaces simply have to objects (I wouldn't naturally call a living thing a surface). The spell does not explicitly state the the surface cannot be a creature but I believe this is implied regardless and so you could not target a Stone Golem with the passwall spell.
This is further supported by the Q/A "Can the Meld Into Stone spell be cast on someone else?", where Jeremy Crawford's tweet (a good source of Rules As Intended) is quoted:
Meld into Stone affects only the caster and the stone the caster touches. If a spell allows you to target others, it defines for you whom you can target.
He has also said in this tweet:
The meld into stone spell works on a stone object or a stone surface, like a wall, floor, or ceiling. An elemental or a golem is a creature, not an object, wall, floor, or ceiling
This supports the idea that a Stone Golem does not count as a surface. The passwall and meld into stone spells use the same wording and so the same logic/idea would apply.
The passwall spell (very likely RAW and definitely RAI) cannot work on a Stone Golem, or any creature for that matter.